Alright man, I just played the MGS saga, let's start this.
SO.
The MGS saga has 2 main sagas. With Big Boss and Solid Snake. You might think they are the same person because they are both named Snake in the games. They are not.
The main character in Metal Gear 1 and 2, and Metal Gear Solid 1, 4 and 2 (for some time) is Solid Snake
The main character in Metal Gear Solid 3, Peace Walker, Ground Zeroes and the upcoming Phantom Pain is Big Boss.
They also look very very similar. I will explain that soon.
Chronologically, the order is MGS 3 -> MGS PW -> MGS GZ -> MGS TPP -> MG 1 -> MG 2 -> MGS1 -> MGS2 -> MGS4
This means you really only have to know about MGS 3 and PW to understand Phantom Pain, but I'll tell you more because of some important details.
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MGS 3 (During the Cold War)
It all started with ANOTHER character called The Boss (there are a lot of bosses in this). The Boss was a woman who trained Big Boss (named Jack initially) . During MGS 3 you play a mission that ends with Jack killing The Boss. A series of complicated twists are revealed, but the gist of the thing is that the USA sent The Boss in a mission to Russia, creating the lie that she "defected" to the soviet union. Unexpectedly, a russian bombed a facility in his own country, to make everyone think The Boss was the culprit. This way, the USA hated The Boss for defecting and Russia hated The Boss for bombing the facility.
Jack, also believing The Boss defected, killed her (orders from the US). Only after this ordeal did he find out that The Boss never betrayed her country, and that the USA used him to get rid of her. He was awarded by the USA and given the title of Big Boss, because he was even "better than The Boss".
Understandably, Big Boss wasn't happy about this, and decided he wouldn't fight for a country, because the country doesn't care about you, or heroes or whatever, proved by how they dealt with The Boss. So he founded Militaire Sans Frontieres, a sort of private army without ideology, country or beliefs. They could be used by the weak to face the strong, they would be the next world power.
It's important to mention that Big Boss's commander was Major Zero in this mission. He also quits. There was also an agent called EVA that helped Big Boss, she rode a bike and loved to show her breasts. That's one of the reasons why Big Boss had sex with her.
The USA saw an opportunity in Big Boss. He was considered the best soldier in the country, a role model, the perfect weapon. Thus they began the Les Enfant Terrible project (The Terrible Children, pardon my french). This project consisted of gathering DNA samples of Big Boss and making clones of him. This is how Solid Snake was born, but hey, I said "clones", in fact, 6 clones, of which 2 survived. Liquid Snake and Solid Snake. This was on purpose. According to Liquid Snake in MGS 1, they created him to hold the recessive genes (hence why is the Nazi party's dream, blonde, blue eyes, all that jazz), in order for Solid Snake to hold the dominant genes and become the strongest soldier possible. More on these shenanigans later.
BACK TO BIG BOSS
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PEACE WALKER (After the Cold War)
Peace Walker was one of the first major missions for MSF (Militaire Sans Frontieres). A professor and his student (a girl named Paz) come to MSF informing BB that Costa Rica has been invaded by a private army and they're doing stuff. BB knows his stuff and realizes the professor is KGB and the private army is CIA. BB does the mission anyway for reasons irrelevant to the major plot. The KGB officer also offers him a base , called the Mother Base, the where MSF does its thing.
BB enters Costa Rica and finds out they are making a giant mech thing called the Peace Walker, the "Ultimate Nuclear Deterrent". What's a nuclear deterrent? Worry not, they explain this to you like 7 times in the game. It's simple, in theory, no one fires nuclear bombs because they fear the retaliation. They fear that this retaliation will start a WW3. But there's a CIA analyst leading this army. His name is.... HOT COLDMAN...I'm serious. No one even questions this. His name is Hot Coldman, but whatevs. He doesn't 100% believe the Nuclear Deterrent Theory, and think humans are flawed, that they could never retaliate knowing that they would start WW3. So Peace Walker basically ALWAYS retaliates, supposedly making the Nuclear Deterrent Theory perfect. To prove this, he wants to be the first to strike...for some reason...I didn't quite understand why, it's not important and basically: Hot Coldman = Cray Cray, as if his name didn't give it away.
Throughout the whole game you're working on a Mech, a bipedal robot called......Metal Gear ZEKE (hey that's the title!)
Long story short, BB destroys the peace walker and puts its nuclear warhead in the Metal Gear ZEKE (yeah because that's a great idea) and all is well.....not. Turns out Paz is an agent working for Major Zero, she hijacks the MGZ (in her underwear for some reason). BB takes MGZ down and Paz falls in the water.
Big Boss is all pumped up and decides MSF will be like, totes cool. They will fight for who needs them, they will face anyone. The Mother Base will be their Outer Heaven.
This is the story of Big Boss up until now.
I'll now fast forward to Solid Snake's adulthood.
In Metal Gear 1, working for FOXHOUND, founded by the "legendary" Big Boss, he invades a base called....Outer Heaven which is making a mech called Metal Gear (hey!) . Lots of things happen and he almost kills Big Boss, since it turns out he owns Outer Heaven, but we knew that. In Metal Gear 2, in another mission in Zanzibar, Solid Snake kills Big Boss.
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MGS 1 (21st Century)
Basically, Solid Snake invades a base called Shadow Moses, occupied by a FOXHOUND team led by Liquid Snake since they're building a.....guess what.... a Metal Gear. (That's basically the apparent mission objective to every game, to stop a Metal Gear from being built, I'm guessing that's how they get away with always using the same name)
Long story short he finds Liquid Snake, who tells him about the Les Enfants Terrible project, that he was supposed to be the inferior soldier. He says that Big Boss was always telling him that he was inferior. Liquid Snake says he wants the world to be like what it was in his father's time (Big Boss), where soldiers, and not information, lead the fight. He will use the Metal Gear for that.
It's also revealed that the medical advisor, Naomi Hunter, injected Solid Snake with FOXDIE, a virus that will eventually kill him. A virus that spread to Liquid Snake, and that's how Liquid died. Solid doesn't know how long he has to live.
Now, what I haven't told you about all this time, is the most ridiculous and strangely important character in the Metal Gear franchise. Ocelot. Ocelot likes guns. A lot, he spins them around, loves reloading them, it's his dream. Hell, Ocelot is actually his last name. His full (code) name is Revolver Ocelot (and what do you know, he uses revolvers!). In fact, just watch this video and laugh a bit:
https://www.youtube.com/watch?v=RHlldvAVBnE
Funny right? Yeah, and that's from MGS 3. So he met Big Boss, and he was stubborn as hell about trying to kill him (trying, he never actually wanted to kill him). We'll see more about his relationship with BB in Phantom Pain.
In MGS 1 he's one of the members of the FOXHOUND team, and he loses his hand to a ninja (yeah but that's another story, I don't think it's relevant, but basically he was Gray Fox a former agent of FOXHOUND working for Big Boss). Ocelot decides it's a good idea to transplant dead Liquid Snake's hand into him arm.
After the credits we hear him making a call. He says "they don't know you're the third clone......Mr.President". So yeah, at the time, the POTUS was the third clone of Big Boss. Solidus Snake.
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MGS 2
In MGS 2 we find out Liquid's personality is taking over Ocelot, and so Ocelot sort of becomes Liquid Ocelot (yeah really). After MGS 1, Ocelot distributed the Metal Gear plans to the world. Now every nation has a metal gear. Metal Gear RAY is the new name. Solid Snake invades a tanker to find out more about it. The tanker is destroyed. Solid Snake is framed for this tragedy and everyone thinks he's a traitor. He sort of dies.
Protagonist switch to Raiden. Now the main character is Raiden.
His mission is in Big Shell, a base formed after the tragedy. Terrorist captured it and have the president (not the third clone, a new president) . Raiden eventually meets the president, who tells him about The Patriots (this is important). The Patriots are like a secret society, they run everything, and they built another Metal Gear ( Arsenal Gear). Where is this Arsenal Gear? "YOU'RE STANDING ON IT". Big Shell, the whole facility is built over this huge Metal Gear, made to fight the Metal Gear RAYs. The Patriots also decide presidents and everything. Full on tinfoil conspiracy. They want to use a software called GW to control the increasing flow of information. If they don't have GW, they can't control the information like they used to.
By the way, Solid Snake also shows up, he survived the tanker thing and he's here to stop Arsenal Gear.
Fast forward. Turns out the whole mission was made to recreate the mission from MGS 1, to make Raiden a soldier as good as Solid Snake, Solidus Snake was Raiden's foster father, he kills Solidus.
Ocelot escapes with the Metal Gear, end game.
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MGS 4
Liquid Ocelot is absolutely fried, now he is totally Liquid Snake.[strike] Because of FOXDIE, Solid is an old man, like really old, FOXDIE accelerated his aging.[/strike] (EDIT: Wrong, it's actually because that's how he was programed as a clone. I think forgetting some details is....understandable). He is Old Snake now.
What happens is nano machines and nano machines and nano machines. Raiden shows up, he's a ninja now.
Solid kills Liquid, destroys the Metal Gear, all that.
It's revealed Solid has 1 year to live.
Solid is obviously a bit sad about this, and decides to kill himself in front of Big Boss's grave. He points a gun at his head and.....
BIG BOSS SHOWS UP
He's alive! Somehow!
He stops Solid from killing himself, and proceeds to drop some exposition.
So he reveals that after whatever happens in MGS 3 happens, he, Major Zero and Ocelot all wanted to free the world of the Cold War ways, they all followed The Boss's ideology. Major Zero created the Patriots. Ocelot did all those things we saw, mostly spinning guns. Big Boss...well, that's what MGS 5 is for. The Patriots, after Zero founded them, became: Zero, EVA ( who is revealed to be the clones's surrogate mother), Big Boss, Ocelot and 2 unimportant people.
Eventually BB and Zero fight over the last M&M and part ways. BB was the symbol of the Patriots, so Zero needed someone to put in all the posters, so he did the Les Enfants Terrible project. This is because after Big Boss left, Zero went cuckoo, which led to the Patriots eventually becoming an AI that manipulates the war economy.
This whole thing happens between MGS 3 and PW, that's why Paz, an agent of zero goes after Big Boss.
He moves to the side and reveals Major Zero, basically a vegetable, stuck in his wheelchair, super old. BB kills him by turning off the machine.
Big Boss tells Solid that Liquid didn't possess Ocelot through is arm, but through drugs and nano machines and other contrived explanations.
Big Boss seemingly dies.
The End.
I sped up the Solid Snake story because it shouldn't be very important for 5.
Throughout the whole story, there is also a tale of bravery and persistence. The constant struggle between Johnny and his sudden bowel movements. In every game, you see Johnny, a lonely soldier fighting a different sort of war. The war against diarrhea. He is triumphant in the end, since he marries a girl, love grand enough to overshadow the constant threat of diarrhea.
(I'm actually serious about that, he shows up a lot)
So, to conclude:
■ Big Boss basically has a private army.
■ He was recently threatened by an agent working for his old commander, Major Zero.
■ Major Zero created the Patriots in between MGS 3 and PW.
■ Big Boss, EVA and Ocelot will eventually join the Patriots. BB eventually leaves and does his thing in PW. After BB leaves, Zero loses his chill.
■ Big Boss has clones, Liquid and Solid Snake who will probably appear in MGS 5.
■ Ocelot knows him, and in the future will be a major player against solid snake.
■ Big Boss's mentor was The Boss, she's like his ultimate role model.
■ Johnny poops a lot.
PS: Chico was a kid he helped in Peace Walker, in case he shows up (he does show up in Ground Zeroes)
PS2: If you end up playing the games, please do hear "genes" as "jeans", it's hilarious. "Don't let your jeans control you!", "Your jeans define you", "I will not be constrained by my jeans", "Our jeans aren't unique!"
That's it. I probably confused you even further, or maybe not, but there is it. MGS ultra fans, feel free to fire at me, I know I'm probably wrong about some things.